Abstract
Abstract The layer compound RbA1F4 investigated by DSC, powder diffraction and linear birefringence is revealed to undergo structural phase transitions at Tc1 = 559.4 K and at Tc2 = 282.1 K. The structures of the three phases are determined by neutron powder profile refinement. The results are consistent with the space group P4/mmm (above Tc1), P4/mbm (from Tc1 to Tc2) and Pmmn (below Tc2). The transition is non-ferroic at Tc1 and improper ferroelastic at Tc2. The linear birefringence measured from 5 K to 630 K showed that the Tc1 phase transition is first order while the Tc2 phase transition is second order. The critical exponent of the order parameter as obtained from Δnab, β = 0.324, does not change on application of uniaxial stress up to 8 MPa.
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